The present embodiments relate to timing measurement in positron emission tomography (PET). Blocks of detectors detect gamma rays emitted indirectly by a positron-emitting tracer. Single photon avalanche diodes (SPADs) or silicon photomultipliers (SiPMs) are solid state photo sensor devices capable of detecting a single photon. Conventionally, a SPAD or SiPM device is built from an avalanche photo-diode (APD) array of microcells on a common silicon substrate. Every APD operates in the Geiger discharge mode, so intrinsically an SPAD or SiPM microcell is a digital device with “0” (switch off) or “1” (switch on) states. SPAD or SiPM may be categorized as analog mode devices, (aSiPM) or digital mode devices, (SPAD, dSiPM).
Using spatially diverse detectors, pairs of gamma rays generated by a same positron may be detected. The pairs of gamma rays travel about 180 degrees apart. To distinguish specific pairs, the coincidence of detected gamma rays is determined. The timing of receipt is used to pair the detected gamma rays. In time-of-flight PET, the timing of receipt indicates a range of locations along the line of response at which the emission occurred. Time-of-flight is used to detect segments of the line of response for more rapid and/or greater resolution reconstruction.
The timing of a detected event is determined using a timing pickoff circuit to determine a time at which the event occurred. Different types of timing circuits have been proposed. The type of timing circuit may depend on the detector. For analog timing, the microcells of the aSiPM device are connected in parallel. Each microcell includes an APD photo sensor and a passive-quenching resistor. The summed anodes and cathodes interface to front-end readout circuits, most of which are outside the aSiPM devices. The circuits may include high gain preamplifiers for the timing channel. Usually, an analog-timing-pickoff (ATP) method is used to obtain the timing-trigger from the comparator-based constant-fraction-discriminator (CFD) or leading-edge discriminator (LED) circuit. After the CFD or LED, the analog pulse is transformed to a “0” to “1” edge-trigger. This trigger is subsequently converted to digital information by a timing-to-digital-converter (TDC) for further processing.
Instead of passively summing all the APD microcells in the aSiPM, SPAD based digital SiPM (dSiPM) uses field-effect transistors (FET), including pMOS and nMOS, to process each microcell digitally—directly obtain the “0” to “1” timing trigger and reset the avalanching microcell by an automatic active quenching circuits. In contrast to an analog SiPM (aSiPM) device, both the APD photo sensors and the logic circuits are integrated in the standard CMOS process. However, conventional SPAD devices for single photon detection have one TDC per microcell. The TDC is a complex device that uses a substantial amount of area in a chip, resulting in less photon detection area. In dSiPM timing detection, the digital triggers from the firing microcells are connected to a trigger network. The trigger network selects a time based on the number of microcells that have fired, and connects this trigger to a TDC. Hence, the SPAD or dSiPM is then restricted to triggering when one microcell (SPAD) or a certain number of microcells (dSiPM) have been fired and not based on the scintillation photon statistics which is an intrinsic physics property in PET timing measurements.